Tensioning apparatuses with actuated pistons having adjustment devices are widespread in prior art and among others used in timing chain drives in internal combustion engines. In that context, the housing forms a pressure chamber in combination with a hollow cylindrical tensioning piston in the piston bore of the housing which is typically filled with a hydraulic medium under pressure and holds a compression spring in order to mechanically pretension the tensioning piston. The adjustment functions of such tensioning apparatuses frequently have detent mechanisms which define a blocking and a sliding direction for the tensioning piston, wherein excessive retracting of the tensioning piston into the housing is prevented by means of a stop while an adjustment of the working range of the tensioning piston is facilitated. For example, such a tensioning apparatus with a simple detent mechanism has been established in DE 190 14 700 A1, wherein the tensioning piston is equipped with a sawtooth profiling into which a ratchet block engages through a window cutout in the housing. In this tensioning apparatus, the free working range of the tensioning piston is limited by the axial freedom of movement of the ratchet block in the housing window.
Another established concept of a tensioning apparatus with adjustment function is disclosed in DE 196 80 418 C1. In that context, the tensioning piston of this hydraulic tensioner has several circumferential detent grooves engaged by the circular clamping section of a detent hook. A stop ring embedded in the housing bore is provided for the realization of the adjustment function at the tension-side end of the piston bore while the blocking of the tensioning piston is realized via a beveled stop surface formed in the housing. A working range sufficient for the tensioning apparatus to develop its damping effect is located in between. In addition to the two clamping sections of the circular clamping region, the detent hook has a U-shaped section and two end legs, each radially extending from the clamping section to the outside and moving in the working range of the tensioning piston in corresponding housing windows between the lower stop surface and the upper stop ring. When adjusting the working range, the detent hook is in an abutting state at the upper stop ring, causing a tensioning piston extending further from the housing to radially spread open the circular section of the detent hook and push the detent hook into the next detent groove of the tensioning piston. If the circular clamping section of the detent hook is in an abutting state at the lower beveled stop surface during a retraction movement of the tensioning piston, the clamping sections of the detent hook will be pressed into the respective detent groove and the further retraction movement of the tensioning piston into the housing will be blocked.
Publication U.S. Pat. No. 7,455,607 B1 describes a generic tensioning system with restoring action, wherein the tensioning piston axially guided in the piston bore of a tensioner housing has a circumferential channel profile holding a detent ring. This detent ring has an essentially closed circular clamping region with two crossing arms which laterally protrude from the ring section through an opening in the housing. A wide circumferential groove in the piston bore of the housing defines the free duty stroke of the tensioning piston towards the detent ring radially protruding at the tensioning piston, respectively forming an upper and a lower stop, wherein the lower stop flank is tilted to clamp the clamping region on the channels of the tensioning piston. The clamping effect of the detent ring on the tensioning piston may be canceled via the arms of the detent ring radially protruding at the housing by compressing the end sections of the arms at the designated handles in order to facilitate restoring the tensioning piston to its original position by stretching open the clamping region, i.e. preventing blocking of the detent ring at the lower stop by stretching open the clamping region from the outside and permitting the tensioning piston to slide into the housing bore without resistance.
Tensioning apparatuses with adjustment devices and a limitation of the retraction movement of a tensioning piston into a housing with a simultaneous definition of the free duty stroke of the tensioning piston are common in different prior art concepts and embodiments and have proven successful in use. Unfortunately, blocking the retraction movement of the tensioning piston is disadvantageous, both during the final assembly of the tensioning systems and particularly during disassembly and subsequent re-installation of the tensioning apparatus during maintenance or service, and for simple tensioning apparatuses sometimes requires the replacement of the tensioning system. This is why there is the requirement to facilitate restoring the tensioning piston, especially for modern tensioning systems with adjustment and blocking devices. However, as the restoring action of the tensioning piston generally opposes the blocking function of the adjustment device, this results in a complex design with additional parts and an individual shape of the required components for most established tensioning apparatuses.
This is why the problem of providing a tensioning system with restoring action for tensioning a flexible drive means, the design of which facilitates a configuration which is as simple as possible and simple handling of a restoring device while also ensuring a reliable function of the adjustment device, is the basis of the present invention.